It is well known that the efficiency of a combustion process is related to the air-to-fuel or oxygen-to-fuel ratio. Conventional types of combustion control systems for packaged boilers are typically referred to as positioning control systems. In positioning control systems, the fuel and combustion air controllers, namely the fuel valve and the fan damper, are mechanically interconnected such that a distinct fan damper position is always associated with a particular valve position. The mechanical interconnection is generally a linkage that incorporates some form of cam that has an adjustable shape which is set at the factory and fine-tuned during boiler start-up, or commissioning, by manual adjustment to give optimum conditions over the load range of the boiler.
During boiler operation a sensor measures a process variable related to the system demand and compares it to a predetermined set-point value. A control signal is then sent to an actuator to modulate the fuel and combustion air controllers to achieve the set-point value. The actuator controls the positioning of the linkage. The linkage generally consists of at least one shaft connected to numerous control rods.
It is common practice to operate boilers with 15 to 30 percent more air than is stoichiometrically required for the complete combustion. The amount of excess air should be closely controlled because too much excess air carries usable heat out of the process and too little excess air may cause the boiler to soot or create an explosive condition. Measurement and control of the amount of excess air is one way to achieve efficient boiler performance. A common method for determining the amount of excess air in the combustion process is to measure the oxygen content of the flue gas exiting the boiler stack.
One problem with prior art control systems is that they do not account for the time lag associated with controlling the amount of excess air in the combustion process. Generally, the time lag may be thought of as the time required for the combustion gases to travel from the burner through the boiler to the stack, where the combustion gases are analyzed for oxygen content. At any given time the amount of excess air detected in the flue gas may not be indicative of the combustion process actually occurring at the burner. Thus, making adjustments to the air flow and fuel flow based on inaccurate readings of excess air can result in inefficient boiler operation.
Therefore, there is a need for a boiler control system that efficiently controls the operation of a boiler or other combustion apparatus. Specifically, there is a need for a boiler control system that responds accurately to the system demand and also accounts for the time lag associated with the boiler combustion process to efficiently control the operation of the boiler.